The disclosures of Japanese Patent Application Nos. HEI 11-55107 filed on Mar. 3, 1999, HEI 11-57572 filed on Mar. 4, 1999 and HEI 11-81450 filed on March 25 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to a fuel-cut control device for fuel supplied to an engine and, more particularly, to a fuel-cut control device and a fuel-cut control method that prevent the engine from going into a stall due to undershooting of engine rotational speed at the time of abrupt deceleration of a vehicle during fuel-cut control.
2. Description of the Related Art
In general, some vehicles having an automatic transmission (including those having a continuously variable transmission) are equipped with a fuel-cut device for cutting off supply of fuel to an engine during deceleration, for example, until the engine rotational speed becomes lower than a predetermined fuel-cut lower limit rotational speed. In such vehicles, since no fuel is consumed during deceleration of the vehicle that does not require an output from the engine, fuel consumption of the vehicle is improved. The greater the fuel-cut range becomes, the greater the effect of improving fuel consumption becomes. Therefore, it is desired that the fuel-cut lower limit rotational speed be lowered.
Thus, for example, according to what is disclosed in Japanese Patent Publication No. HEI 7-12807, while a fuel-cut device cuts off the supply of fuel to an engine, the engine can be operated by engaging (connecting) a lock-up clutch for performing mechanical connection and disconnection between the engine and a drive wheel and by transmitting a driving force of the vehicle to the engine through the lock-up clutch. In this construction, while the fuel-cut device cuts off the supply of fuel to the engine, the lock-up clutch is engaged (including a slipping state based on half engagement) so that a rotational force based on the running of the vehicle is transmitted to the vehicle. Therefore, at the time of deceleration, the rotational speed of the engine gradually decreases in accordance with a vehicle speed. Thus, the fuel-cut range is enlarged. As a result, the fuel consumption of the vehicle is improved in comparison with the case where the lock-up clutch is released (disconnected) as soon as the vehicle starts to decelerate.
Further, if the rotational speed of the engine has become equal to or lower than the fuel-cut lower limit rotational speed, the lock-up clutch is disconnected (released) and the engine is set free from the drive wheel (connected to the drive wheel only through fluid in a torque converter). Then fuel injection for the engine is resumed and the engine is self driven, so that idling rotation of the engine is maintained.
When an accelerator pedal is depressed, the engine output is increased and the lock-up clutch is connected again, so that the output of the engine is transmitted to the drive wheels.
As described above, if the vehicle decelerates to a predetermined speed and fuel-cut operation is terminated, the engine is mechanically disconnected from the drive wheels by disconnecting the lock-up clutch, and fuel injection is resumed. In this case, as shown in a timing chart in FIG. 4, if an engine control device recognizes a timing for terminating the fuel-cut operation by making a calculation based on a vehicle speed, an engine load, an engine rotational speed and the like, the fuel-cut state is switched from ON to OFF (0) and the lock-up state is switched from ON to OFF (0). Then the lock-up clutch is disconnected and the engine is mechanically disconnected from the drive wheels (actually from a transmission such as a CVTxe2x80x94Continuously Variable Transmissionxe2x80x94or the like). Furthermore, at this moment, based on the fact that the fuel-cut state has been switched from ON to OFF (0), fuel injection for the engine is resumed so as to operate the engine at a predetermined rotational speed (idling rotation).
As shown in the timing chart in FIG. 4, the fuel-cut device or the like requires a predetermined length of calculation time in order to make a calculation for recognizing a timing for terminating the fuel-cut operation (e.g., a calculation in which the vehicle speed V is used as a trigger). Therefore, there is a time lag from detection of the vehicle speed V to a change (from ON to OFF) in fuel-cut state or in lock-up state. As indicated by a broken line in FIG. 4, when the vehicle decelerates in a normal running state, the fuel-cut state or the lock-up state is turned OFF before the engine rotational speed becomes lower than a threshold value T despite the presence of the time lag, so that fuel injection is resumed. Hence, the engine rotational speed can be brought back to a predetermined rotational speed (e.g. an idling rotational speed) without becoming lower than the threshold value T (see the broken line in FIG. 4).
However, if the driver has abruptly depressed a brake pedal for some reason during the fuel-cut control, namely, if the vehicle has abruptly decelerated (see a solid line in FIG. 4), and the fuel-cut state or the lock-up state is not OFF in the course of a calculation in which the timing for terminating the fuel-cut operation is recognized using the vehicle speed V as a trigger, the lock-up clutch remains connected and the engine rotational speed abruptly decreases with a decrease in rotational speed of the drive wheel. As a result, even if the lock-up clutch has been disconnected or fuel injection has been resumed after the calculation, the engine rotational speed cannot be restored (see the solid line in FIG. 4), which causes a problem of engine stall. Especially in the case where the fuel-cut range is enlarged to improve fuel consumption (the rotational speed upon termination of the fuel-cut operation is lowered), the aforementioned problem of engine stall resulting from abrupt deceleration is acute.
The present invention has been made in consideration of the aforementioned problem. It is an object of the present invention to provide a fuel-cut control device that is capable of preventing an engine from going into a stall due to the undershooting of engine rotational speed when a vehicle abruptly decelerates during fuel-cut control.
In order to solve the aforementioned problem, a fuel-cut control device according to a first aspect of the present invention includes an engine, a torque converter having a lock-up clutch for performing operations of mechanical connection and disconnection of power between the engine and a drive wheel, a fuel-cut device for performing fuel-cut control wherein supply of fuel to the engine is suspended until the vehicle reaches a predetermined running state with the lock-up clutch being connected during deceleration of the vehicle, an abrupt deceleration detector for detecting abrupt deceleration of the vehicle, and a forcible restorer for suspending the fuel-cut control and making forcible restoration of control out of the fuel-cut control when the abrupt deceleration detector detects the abrupt deceleration.
It is to be noted herein that the predetermined running state of the vehicle means a certain state of the vehicle that can be determined, for example, based on a vehicle speed, an engine rotational speed and the like. In this construction, if abrupt deceleration of the vehicle has been detected, fuel-cut control is suspended immediately and forcible restoration of control out of the fuel-cut control is made. Therefore, even when the engine rotational speed abruptly decreases, it is possible to disconnect the lock-up clutch and resume fuel injection before the engine rotational speed becomes lower than a predetermined value. Thus, the engine rotational speed can be restored, and the engine can be prevented from going into a stall due to the undershooting of the engine rotational speed. The undershooting of the engine rotational speed means a phenomenon wherein the engine rotational speed drops way below a predetermined rotational speed. If the engine rotational speed becomes equal to or lower than the predetermined rotational speed, the engine is stopped.
The abrupt deceleration detector may be designed to detect abrupt deceleration of the vehicle based on at least one of an operational amount and an operational speed of a brake pedal of the vehicle.
According to this construction, abrupt deceleration of the vehicle can smoothly be recognized without making a complicated calculation. Therefore, forcible restoration of control out of the fuel-cut control can be made more swiftly.
The abrupt deceleration detector may be designed to detect abrupt deceleration of the vehicle based on an actuating signal of a brake assist system or an anti-lock brake system.
According to this construction, abrupt deceleration of the vehicle can smoothly be recognized making use of a result detected by an existing system. Therefore, forcible restoration of control out of the fuel-cut control can be made more swiftly. Also, since there is no need to add any component for detecting abrupt deceleration of the vehicle, engine stall at the time of abrupt deceleration can be prevented at a low cost.
Furthermore, the fuel-cut control device of the present invention may include an engine output controller for controlling an engine output by adjusting an amount of fuel supplied to the engine and may be designed such that when the vehicle reaches a predetermined running state and restoration of control out of fuel-cut operation by the fuel-cut device is made simultaneously with disconnection of the lock-up clutch, the engine output controller increases an engine output by a predetermined amount with respect to an idling maintenance output from a time of restoration of fuel injection and then restores the idling maintenance output.
According to this construction, when disconnecting the lock-up clutch, the torque can be increased because of an increase in engine output. Therefore, it is possible to reduce a torque discrepancy at the time of disconnection of the lock-up clutch, whereby it becomes possible to attenuate a shock delivered to the vehicle and to prevent the engine rotational speed from undershooting.
Further, the fuel-cut control device of the present invention may be designed as follows. That is, the engine output controller makes restoration of control out of the fuel-cut operation in a gradually increasing manner such that the engine output becomes greater than an idling maintenance output, a predetermined time prior to disconnection of the lock-up clutch after attainment of a predetermined running state by the vehicle, and restores the idling maintenance output after lapse of a predetermined time.
According to this construction, the control for increasing an engine output is gradually started prior to disconnection of the lock-up clutch. Therefore, the torque discrepancy is completely eliminated at the time of disconnection of the lock-up clutch, and the engine rotational speed changes smoothly. As a result, it is possible to attenuate a shock (at the time of restoration of control out of the fuel-cut operation) delivered to the vehicle and to inhibit engine stall securely and effectively.
Furthermore, the fuel-cut control device of the present invention may further include a delay detector for detecting a delay of disconnecting operation of the lock-up clutch when the vehicle reaches a predetermined running state and restoration of control out of the fuel-cut operation by the fuel-cut device is made simultaneously with disconnection of the lock-up clutch, and an output amplifier for increasing, based on a result detected by the delay detector, an engine output from an idling maintenance output.
According to this construction, if there is a delay in disconnection of the lock-up clutch, the engine output is increased from the idling maintenance output. Therefore, the engine torque increases and the engine rotational speed rises. As a result, it is possible to avoid engine stall.
According to a fuel-cut control method of the present invention, a delay of disconnecting operation of the lock-up clutch is detected when the vehicle reaches a predetermined running state and restoration of control out of the fuel-cut operation by the fuel-cut device is made simultaneously with disconnection of the lock-up clutch, and an engine output is increased from an idling maintenance output upon detection of a delay of disconnecting operation of the lock-up clutch.
According to the thus-constructed fuel-cut control method of the present invention, if abrupt deceleration of the vehicle has been detected, the fuel-cut control is suspended immediately and forcible restoration of control out of the fuel-cut control is made. Therefore, even when the engine rotational speed abruptly decreases, it is possible to disconnect the lock-up clutch and resume fuel injection before the engine rotational speed becomes lower than a predetermined value. Thus, the engine rotational speed can be restored, and the engine can be prevented from going into a stall.